This invention relates to apparatus for forming consolidated pucks of granular material from a slurry by applying pressure to the slurry in a hydraulic press to remove the liquid phase.
Commonly owned U.S. Pat. application Ser. No. 330,227 filed on Dec. 14, 1981 in the names of David W. Coate and Joseph G. Selmeczi and entitled "Apparatus and Process for Dewatering Fine Granular Material" which is a continuation in part of U.S. Pat. application Ser. No. 226,691 filed on Jan. 21, 1981 but now abandoned, discloses apparatus for dewatering granular materials in which a press is provided with drain openings which are one to five times larger in their smallest dimension than the size of the largest particle in the slurry. While it might appear at first that such a press could not dewater the granular material since the granules being smaller than the slots would just be expressed with the liquid phase, in fact after an initial loss of some granular material, the granules form bridges across the slots which act as filters to retain the bulk of the granular material. Such apparatus is self-cleaning and will not clog because the particles are not large enough to become lodged in the drain openings and the initial flow of liquid removes any residue of the bridges which are, for the most part, removed as part of the consolidated puck.
In such a press, the rate of movement of the piston must be slow enough that the granular material does not just blow through the drain openings without bridging. This is a limiting factor on the rate at which pucks can be formed. We have also found that bridging does not occur simultaneously in all the slots and that as a result, an unequal transverse force is imposed on the walls forming the sides of adjacent drain openings when one is bridged and the other is not. This requires that the structure between the drain openings be strong enough to resist these transverse forces which in turn places a limitation on the number of drain openings and thereby limits the rate of discharge of the liquid phase.
While it has been suggested in the past that a more compact press can be made by placing opposing press units on opposite sides of a central planar member, in the past the object has been to keep the forces generated by the two press units equal so that no net force was applied to the central planar member. This was accomplished by simply supplying pressurized hydraulic fluid to the opposed press units from a common source. While this permits the use of a relatively thin, lightweight central planar member, it increases the time required to form the pucks of granular material. Due to variations in the consistency of the slurry, the pressure versus time function for each press unit supplied with hydraulic fluid at a constant flow rate is not a smooth curve but exhibits positive and negative deviations from the theoretical function. By connecting both press units to a common source of pressurized hydraulic fluid at a given flow rate, the combined units are always operating at the pressure dictated by the press unit which is at the lowest point on its pressure versus time curve. Thus, it takes longer for the two interconnected units to produce two pucks than it would if each unit was operated separately where at least some of the time the pressure would be above the expected value.
In order to overcome these limitations of the prior art presses while making use of some of their advantages, the present invention has the following objectives.
It is the principle object of the present invention to provide apparatus which will rapidly and efficiently remove the liquid phase from slurries containing granular material.
It is a subordinate object of the invention to achieve the above objective by utilizing a press which has drain openings which are one to five times larger in the smallest dimension than the size of the largest particle in the granular material in the slurry.
It is another object of the invention to provide such a press in which the flow of hydraulic fluid to the press is increased after the granular material has formed bridges over these drain openings.
It is yet another objective of the invention to provide a press which has such drain openings on the piston as well as in a fixed wall of the press cavity to permit more rapid removal of the liquid phase.
It is an additional object of the invention to provide such a press which utilizes a sliding mold cavity to form the pucks of granular material.
It is also an object of the invention to provide such a press in which the sliding mold is substantially fully supported by the associated piston.
It is still another object of the invention to provide such a press with interchangable parts which allow for the selection of drain opening size and puck size suitable for the particular slurry being pressed.
It is a further object of the invention to provide a compact press satisfying the above objective with two opposed presses forming separate pucks simultaneously on two sides of a planar member.
It is a still further object of the invention to allow the pressure in the two presses on opposite sides of the planar member to vary within preselected limits which provide limited balancing of the forces on opposite sides of the planar member while decreasing the time required to form two pucks of granular material.